*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}
body{
    position: relative;
    width: 100%;
    height: 100vh;
    

}
.in,.out{
    position: absolute;
    width: 200px;
    height: 200px;
    border-radius: 50%;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);

}
.in{

    border: solid 1px black;
} 
.out{
    width: 180px;
    height: 180px;
    border: solid 1px black;
    background-color: #3498db;
}
.out::after{
    content: "";
    position: absolute;
    width: 270px;
    height: 270px;
    border-radius: 40%;
    background-color: #fff;
    top: 0;
    left: 50%;
    transform: translate(-50%,-60%);
    animation: animate 5s linear infinite;
}
@keyframes animate {
    /* 可以通过调整y轴的阈值来使水面的水平位置 */
    100%{
        transform: translate(-50%,-60%) rotate(360deg);
    }
}

